class Point {
   Point() {};
   pt P = new pt();     // mouse position 
   pt L = new pt();     // last mouse position 
   vec V = new vec();   // mouse speed 
   vec A = new vec();   // mouse acceleration
   float l = millis();  // time of last update   
   //
   void update(float xm, float ym) {
      L.setTo(P);
      P.setToMouse(xm,ym);
      float t=millis()-l;             // time since last update
      t=1;
      vec U =  L.vecTo(P); U.div(t);  // new speed U = LP/t;
      A.setTo(U); A.sub(V); A.div(t); // new acceleration A=(U-V)/t
      V.setTo(U);
      l=millis();
      };
   } 
 
